In TDA, the principal was never taxed…so when you take the money out of that account, it is taxed at that point along with interest (meaning entire amount).
However, in tax deferred gains, the principal is already after tax amount. So, only interest is taxed and hence the formula adds back the taxes on principal.
Wrong, if B=0, the basis (principal) would be also taxed what is, IMO rare situation in the real life but Tax Authority may argue that they do not recognize cost basis as tax exempt in which case whole earning amount is taxable like it was built up from zero.
Was reviewing Example 10 from the tax reading in CFAI and was having the same question.
Within that example, 2) the taxable account with deferred capital gain had a higher FV than 3) the TDA.
So what you guys are saying is 2) is higher because 3) taxes the principal, but 2) need not be taxed again because it is already after tax to begin with?